Does River Flow Regulation Change the Composition and Diversity of Riverine Vegetation

Abstract

River regulation as a result of construction of dams caused global-scale ecological changes toriverine ecosystems. Studies have shown that vegetation alterations may take place due tochanges in the water flow, flooding frequencies, and fluctuating water table associated due todam construction. The Accelerated Mahaweli Project (AMP) which started in 1970’sinvolved construction of five major dams across the Mahaweli River. However, no study hasbeen undertaken so far to assess the dam-induced impacts on downstream riverine forests.The present study was aimed to investigate any dam-induced impacts on the riverinevegetation in the immediate downstream areas of the AMP. In order to achieve this objective,a stretch of Mahaweli River with a regulated water flow (in Minipe area) and a tributary ofMahaweli river with an unregulated water flow Badulu Oya was selected for the study. Acomplete vegetation sampling was carried out using eight 10 m x 30 m belt transects laidperpendicular to the river flow. Each belt transect was divided into three 10m x 10m zones(hereafter named as lower, mid and upper slopes). The vegetation was quantified under threecategories canopy vegetation, understory and ground vegetation. Paired t-test (95% CI) inMinitab 16.0 revealed that the canopy vegetation abundance in the lower slope issignificantly higher (p ≤ 0.039) at Badulu Oya than that of Minipe. However, no significantdifference was observed in other two zones in between the two rivers. The lower slopeshowed similar trend in terms of canopy vegetation diversity though it was not significant.Terminalia arjuna and Ficus racemosa dominated the canopy vegetation in Badulu Oya andMinipe, respectively. Both abundance and diversity of the understory vegetation wassignificantly higher at Badulu Oya than that of Minipe and this effect is more prominent inthe lower- and mid-slopes than in the upper slope. Dimorphocalyx glabellus dominated theunderstory vegetation at Badulu Oya while at Minipe it was Cipadessa baccifera. Riverinevegetation in both rivers was dominated by native species, however no significant differenceobserved in the abundance of invasive plants. Interestingly, at Badulu Oya the abundance ofthe riverine vegetation decreased towards the upper slope but at Minipe no such trend wasobserved. The preliminary findings of the research indicate that the regulation of riversthrough damming can have long-term impacts on the riverine vegetation by changing itscomposition and diversity. However, further studies will authenticate this inference.